On the world market there may presently be observed a shortage of proteins, especially in the form of meat, milk, and corn, and this shortage is still increasing. For this reason, alternate sources of proteins for human nutrition are being sought. One of the proposed sources comprises the protein of single-cell microorganisms (single-cell protein) such as yeasts, bacteria, algae and the like. The advantage of the single-cell proteins consists in the possibility of automating their production, and also in the use of waste saccharides or materials of a non-agricultural origin as the starting material. The growth rate of these microorganisms is much higher than that of macroorganisms as it may be seen from Table 1, below.
TABLE 1 ______________________________________ (according to Proces Biochem. January 1971, p.41) Time necessary to double Organism the weight of cells ______________________________________ bacteria and yeasts 20-120 minutes fungi and algae 2-6 hours green plants 1-2 weeks chickens 2-4 weeks pigs 3-5 weeks cattle 4-10 weeks ______________________________________
The use of microorganisms as the source of proteins in nutrition is accompanied by some difficulties such as the usual bad smell of microorganism (e.g. yeasts), the lowered digestibility due to cell walls, the high content of nucleic acids which are deleterious to the human organism, the lowered possibility of converting the concentrate into a suitable form, lack of consistency and the like.
There are presently known procedures whereby some of these disadvantages may be eliminated. The typical yeast smell may be thus removed e.g. by extraction with a hot edible oil (Paper presented at the VIIIth Inter. Congr. of Nutrit., Prague, Aug. 28 - Sept. 5, 1969). Nucleic acids in intact cells are removed by activation of the intracellular nuclease on heating the suspension of intact cells at an elevated temperature, e.g. 75.degree.C, for a very brief time, e.g. 15 seconds (S. B. Maul, A. J. Sinskey, and S. R. Tannenbaum in Nature 228, 181 (1970); A. Imada, A. J. Sinskey, and S. R. Tannenbaum in Biotechn. Bioengin. XIV, 103, (1972); and S. Ohta and coworkers in Applied Microbiology, Sept. 1971, (p. 415-421) the isolation of proteins from intact cells has been performed by extraction with 85% aqueous formic acid (Brit. Pat. Nos. 1,224,172; 1,224,173; and 1,224,174), with sodium carbonate (Brit. Pat. Nos. 1,234,173; and 1,234,174) or the proteins may be exracted with 8-10 molar aqueous urea for 24 hours (French Pat. No. 1,552,867).
Some preparations of the microbial protein from microorganisms consist in the mechanical disruption of cells and the subsequent extraction with sodium hydroxide at room temperature for about two hours. (Biotechnology and Bioengineering 1970, p. 947). Simultaneous extraction with neutral agents such as sodium chloride and disruption has been disclosed in Czechoslovak Pat. No. 140,926.
The disadvantage of all the known procedures based on extraction of intact cells consists in the long action of extracting agents due to the necessity of penetration of proteins through the cell wall. Since the effectiveness of extraction is very low, this process has not been so far used in industry. Industrial applications have been made possible by newer procedures which include the disruption of microorganisms, but a suitable degree of disruption may be achieved only by repeated disruptions in a homogenizer. (P. J. Hetherington in Trans. Instn. Chem. Engers. 49,142 (1971); Lars Edebo in Journal Biochem. Microbiol. Technol. Eng. 11(4),453 (1963).